1. Field of the Invention
This invention relates to compact zoom lenses suitable for use in photographic devices.
2. Description Relative to the Prior Art
A number of kinds of zoom lenses are known for use in single lens reflex (SLR) cameras. Many such SLR camera lenses are so-called "short" zoom lenses that are comprised of a front negative component and a rear positive component. An example of such a zoom lens is disclosed in U.S. Pat. No. 4,198,128. Although this zoom lens itself is relatively short, it has a long back focal length. The long back focal length provides the necessary space in the SLR camera to allow for insertion of a mirror. However, the length from the first lens surface to the image forming or focal plane is relatively long. Thus, even though the lens itself may be short, a camera embodying the lens is inevitably bulky because of the long back focal length.
A non-SLR camera (sometimes called a "viewfinder" camera) does not require a long back focal length. There is thus an opportunity to provide a less bulky camera by designing a compact zoom lens with a short back focal length.
U.S. Pat. No. 4,936,661 describes a zoom lens having a short back focal length with negative, positive and negative units from front to rear. The negative unit closest to the image provides the majority of the change in focal length. In the less complex examples the front negative and positive units do not move relative to each other. In these examples the chromatic aberration could not be sufficiently corrected to get both a high aperture and a broad zoom range.
A zoom lens system disclosed in Japanese Kokai (unexamined Publication) No. 56-128,911, includes a first positive lens group and a second negative lens group which move relatively to vary the distance therebetween and to vary the focal length of the system for zooming. This zoom lens system, while being sufficiently short in overall length when adjusted for wide-angle photography, is limited to a zooming ratio of approximately 1.5, which is less than that usually available for an SLR camera.
Another zoom lens, disclosed in U.S. Pat. No. 4,787,718, comprises a first negative lens group, a second and a third positive lens group, and a rear negative lens group. It has a zooming ratio of about 2.times. and a sufficiently shortened overall length. This lens system has four moving groups, and although the distance between the first and third lens group is maintained during zooming, the two positive groups move at different speeds, and the relative distance between them changes. This zoom lens system has eight glass elements.
A zoom lens disclosed in U.S. Pat. No. 4,830,476 comprises two moving lens groups--a front positive lens group and a rear negative lens group. This zoom lens is relatively compact and has a zooming ratio of about 2.times.. Both the front group and the rear group have one positive and one negative lens element. Although this zoom lens has only four lens elements, in order to correct aberrations sufficiently, all five embodiments use at least three, and sometimes four, gradient index elements.
A zoom lens disclosed in U.S. Pat. No. 4,818,081 uses a configuration similar to the one disclosed in '476. As in '476, the front lens group of the lens system disclosed in U.S. Pat. No. '081 contains negative and positive subgroups which stay together during zooming. In addition, as in the zoom lens disclosed by '476, the rear zooming lens unit contains a positive, as well as a negative lens. This lens has a zooming range of 1.5 to 1.6.times. which is less than generally desirable for an SLR camera.